Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as an energy source. Among these secondary batteries, lithium secondary batteries having high energy density and voltage are commercially available and widely used. The lithium secondary batteries generally use a lithium transition metal oxide as a cathode active material and a graphite-based material as an anode active material.
However, the anode formed of the graphite-based material has a maximum theoretical capacity of only 372 mAh/g (844 mAh/cc), thus suffering from a limited increase of capacity thereof. Consequently, such a graphite-based anode is incapable of carrying out a sufficient role as an energy source for next-generation mobile equipment undergoing rapid development and advancement. Further, lithium metals, studied for use as the anode material, have a very high energy density and thus may realize a high capacity, but raise problems associated with safety concerns due to growth of dendrites and a shortened cycle life as the battery is repeatedly charged/discharged. In addition, carbon nanotubes (CNTs) have been used as an anode active material, but have suffered from problems such as low productivity, high costs and low initial efficiency of less than 50%.
In recent years, a number of studies and suggestions have been proposed as to silicon, tin or alloys thereof, as they are known to be capable of performing reversible absorption (intercalation) and desorption (deintercalation) of large amounts of lithium ions through the reaction with lithium. For example, silicon (Si) has a maximum theoretical capacity of about 4020 mAh/g (9800 mAh/cc, a specific gravity of 2.23) which is substantially greater than the graphite-based materials, and thereby is promising as a high-capacity anode material.
However, upon performing charge/discharge processes, silicon, tin or alloys thereof react with lithium, thus undergoing significant changes of volume, i.e., ranging from 200 to 300%, and thus repeated charge/discharge may result in separation of the anode active material from the current collector, or significant physicochemical changes at the contact interfaces between the anode active materials, which are accompanied by increased resistance. Therefore, as charge/discharge cycles are repeated, the battery capacity sharply drops, thus suffering from a shortened cycle life thereof. For these reasons, when a conventional binder for a graphite-based anode active material, without any special treatment or processing, is directly applied to a silicon- or tin-based anode active material, it is impossible to achieve desired effects. In addition, when an excessive amount of a polymer as the binder is used to decrease volume changes occurring upon charge/discharge cycles, the electrical resistance of the anode is increased by an electrical insulating polymer used as the binder, which consequently results in problems associated with a reduced battery capacity and a decreased charge/discharge speed.
In order to cope with such problems, there is an urgent need for the development of a binder having low electrical resistance while exhibiting adhesive strength and mechanical properties sufficient to withstand large volume changes of anode active materials occurring during a charge/discharge process in lithium secondary batteries using silicon- or tin-based anode active materials. In addition, conventional graphite-based lithium batteries also require high-speed charge capability via further enhanced conductivity of the binder.
On the other hand, as an attempt to use conventional carbon nanotubes as an anode mix (a mixture of an active material, a conductive material, a binder and the like) of a lithium secondary battery, Japanese Patent Laid-open Publication Nos. 2004-319186 and 2005-004974 disclose incorporation of the carbon nanotubes as the conductive material into the anode mix, thereby improving the conductivity of the anode. However, to the best of our knowledge, no case has been known to the art in which the carbon nanotubes were used as the binder of the anode mix. Furthermore, the carbon nanotubes are highly expensive materials, and therefore incorporation thereof in a content required as the conductive material into an electrode mix leads to the fundamental problem associated with significantly increased production costs of the batteries.